Any curtains that are washable and made from natural fibres can be dyed at home in your washing machine. You can cover up stains on your old curtains or match them to a new decorating scheme. They don't have to be white; any light colour can be overdyed. These instructions will dye about 3.63kg. of fabric.
Check the fibre content of your curtains. Only natural fibres can be dyed by this method. Natural fibres include cotton, linen, rayon, hemp and silk. If your curtains are a blend of natural fibres and synthetic fibres, only the natural fibres will take the dye, which may result in uneven, blotchy or pastel colours. It is harder to guarantee specific results with blended fabrics, but you may still be able to create something beautiful.
Choose dye colour carefully. If your curtains are already coloured, the new colour will blend with the existing colour rather than covering it. For example, if you have yellow curtains and you dye them with red dye, you will have orange curtains.
To test a swatch in advance, cut a small piece of fabric from an inside seam of the curtains. Stir 1/8 tsp dye into 1 tbsp salt, then add 1 cup of very hot water and stir until dye and salt are dissolved. Add 1/2 tsp soda ash and mix thoroughly. Add fabric swatch and stir well, then let sit for at least 30 minutes. Wash swatch thoroughly in water and dish detergent until the water runs clear, then let dry. This should provide a good idea of the results you will get from this method.
Wash curtains in hot water with a squirt of dish detergent to remove any dirt or grease. Remove curtains from washing machine.
Add 20 cups salt and 1/2 cup fibre-reactive dye to the washing machine and fill machine on the hottest setting. Agitate to mix the dye and dissolve the salt. When dye and salt are thoroughly blended into the water, add soda ash and agitate again to dissolve the soda ash thoroughly.
Add curtains to washing machine and agitate. For deepest colours, agitate at least 30 minutes. This may require you to occasionally reset the wash cycle on the machine.
After initial wash cycle is finished, run another wash cycle in hot water with a squirt of dish detergent. Rinse in cold water. Repeat rinse until water is clear, then remove from washer. Hang to dry.
Fibre-reactive dyes, usually sold under the Procion trade name, are dyes that form chemical bonds with the fabric. They are more colour-fast and resistant to fading than the dyes usually sold in hobby stores, such as Rit. For curtains that will be exposed to sunlight, it's important to use fibre-reactive dyes to avoid fading. Soda ash (sodium carbonate) can be purchased at any online dye supplier or in many supermarkets as "washing soda." If you buy washing soda, be sure to check that it is pure sodium carbonate and does not contain added scents or detergents. Add a few drops of dish detergent to your dye bath to get more even results. Use more dye for a deeper colour, less dye for pastels. This recipe will dye about 3.63kg. of fabric. For exact results, weigh your curtains and adjust the amounts of dye and chemicals accordingly.